1. Field of the Invention
This present invention relates to an output stage. More particularly, the present invention relates to an impedance control apparatus for adjusting the output impedance of the output stage.
2. Description of the Prior Art
Please refer to FIG. 1; FIG. 1 shows the schematic diagram of the conventional output stage circuit 60 capable of adjusting the output impedance of an output stage 65. The output stage 65 comprises two adjustable resistor arrays 66-67 which are the same. One replica resistor array 68 that is replicated from the resistor array 66-67 is used to form an adjusting loop. The potential of the replica resistor array 68 is compared with a reference potential in order to produce a comparison signal, then the control logic 36 generates a control signal according to the comparison signal to adjust the output impedance of the output stage 65 of the output stage circuit 60, so as to obtain the preferred impedance of the output stage 65.
Because of the differences in the production process of electronic elements, which cause the drifting in the electronic characteristic of electronic elements (for example: resister, current source), it is difficult to replicate the exact same elements with the same characteristics, thus affecting the accuracy of the output impedance. Besides, the power consumption and the cost of the conventional output circuit are higher because the conventional output circuit needs these replica electronic elements.